We have obtained evidence that developmental exposure to the AhR-specific ligand TCDD (dioxin) affects epigenetically-regulated events leading to long-lasting defects in the offspring's response to influenza A virus infection. Defects include reduced expansion and differentiation of virus-specific CD8+ T cells, impaired production of the antiviral cytokine IFN?. Objectives/Hypothesis: The purpose of this project is to understand how environmental signals delivered via AhR during fetal and neonatal development cause long-lasting impairment of CD8+ T cell responses to infection. Although epigenetic mechanisms are known to regulate gene expression in immune cells, the idea that inappropriate AhR activation during development causes permanent functional changes via an epigenetic mechanism is a novel paradigm. The overall hypothesis for the proposed studies is that inappropriate AhR activation during development interferes with the programming of the immune system via epigenetic mechanisms, resulting in permanent defects in gene expression that lead to long-lasting reductions in CD8+ T cell function. Specific Aims:1) To identify the developmental and lineage-specific targets of AhR that cause long-lasting reductions in the capacity of CD8+ T cells to respond to infection, we will determine the contribution of reprogramming in specific hematopoietic lineages using a combination of progenitor cell transplantation, lineage-specific gene ablation, and adoptive transfer. 2) To identify gene targets whose expression is differentially silenced or activated by developmental exposure to dioxin, we will define AhR-mediated changes in CpG methylation and covalent histone modifications of the ifng locus, and use novel genome-wide approaches to identify additional epigenetically-regulated genes that are affected. 3) To identify epigenetic regulatory pathways modulated by AhR we will use pharmacological and genetic approaches to determine which DNA methyltransferases are affected and whether these changes contribute to impaired CD8? T cell functions, such as impaired IFN? production following infection. Significance: The proposed studies address an area that is relevant to global human health but has received inadequate attention. Influenza and other viral infections continue to pose significant global health threats. Evidence points to prenatal and early life exposure to pollutants as overlooked contributors to poorer clinical outcomes following infection. Moreover, the idea that AhR impacts epigenetic programming is innovative and has broad biological significance, as AhR plays a role in normal hematopoiesis, and developmental exposure to AhR ligands deregulates many aspects of immune function. Moreover, TCDD and other AhR ligands disrupt development of other tissues, thus findings from these studies will help us better understand how AhR ligands impact cells throughout the body, and will provide new information regarding the normal physiological role of AhR.